I have a player that uses jump drive memory, so I keep a hand-full of 64Ms in the gym bag. They hold about 1 hour each.

Anyway, to the point; I find that for me the most effective workout music is either hard rap (NWA, Dre, Ice Cube/T, etc) or hard metal/thrash (Rammstein, Pantera, Damageplan, etc). The EFD they play at the health club is just utter crap.